1. Field of the Invention
This invention relates to shift control systems for keyboards, and more particularly to a shift control system suitable for general keyboards, wherein the shift keys are disposed in a common matrix arrangement with the general keys.
2. Description of the Prior Art
Currently, general keyboards of the scan type sense the ON and OFF condition of the key switches arranged as a matrix by means of a central processing unit or an LSI circuit for controlling the keyboards as shown in FIG. 1. As shown in FIG. 1, the prior art keyboard has a keyboard controller 1, scan lines 2, a decoder 3, return lines 4, a keyboard matrix 5, and sense lines for the ON and OFF states of the shift keys. The keyboard controller 1 scans the key switches arranged in a matrix to sense the ON and OFF states, and if one of the keyswitches in "ON", the controller repeats the scan to confirm the "ON" state of the switch and stores the address X and Y of the "ON" switch in a buffer memory and requires an interrupt to the CPU. Currently, most keyboards have fifty keyswitches. However, the above method restricts the possibility of reducing the number of key switches to display the same number of characters and has the disadvantage of complicated operation if the number of keys are increased to accomodate more display characters.
One solution of the aforesaid problem has been to use shift keys to select categories of alphabets, numerals and special symbols enabling one or more key switch to generate three different display characters. In this case, the keyboard must have a sense line 6 for each shift key and a separate register for each sense lines 6, so that the circuitry becomes complicated, resulting in high manufacturing cost. Furthermore, the keyboards are not conducive for general use, because it is difficult to change the key arrangement after it has been designed. Therefore use of the prior art keyboards with various devices requires design of special circuits for each device.